1. Field of the Invention
This invention relates to coating resins with attached light stabilizer functional groups. More particularly, it relates to acrylic resins suitable for coating applications which contain ultraviolet light absorbing (UVA) stabilizers and/or hindered amine light stabilizers (HALS) chemically bound to the acrylic resin.
This invention provides novel acrylic resins with chemically attached light stabilizing groups, wherein improved light stabilizer performance has been obtained by means of binding the light stabilizer group to the acrylic resin.
This invention further relates to the use of these acrylic resins with chemically bound light stabilizer groups in coating compositions for the stabilization of the coating against thermal and/or photooxidative degradation.
This invention provides a cost effective method of improving the durability and weatherability of coatings, and provides improved product economics over prior art light stabilization methods for coatings.
2. Description of the Prior Art
Synthetic polymers, including organic coatings and films, are sensitive to and degraded by the deleterious effects of weather components such as heat, sunlight, and moisture. Light and heat stabilizers such as ultraviolet light absorbing (UVA) stabilizers and hindered amine light stabilizers (HALS) are used to inhibit such deterioration.
Ultraviolet light absorbing stabilizers act by absorbing the harmful UV light energy of sunlight and converting it into thermal energy at low enough levels that the heat is much less deleterious than the radiation. Representative UVA stabilizers include derivatives of 2-hydroxybenzophenones, 2-(2-hydroxyphenyl)benzotriazoles, and the oxalanilides, for example.
Hindered amine light stabilizers (HALS) derived from 2,2,6,6-tetraalkylpiperidine compounds do not absorb ultraviolet light, but stabilize by reacting with radicals formed from degradative processes, thus preventing harmful side reactions, such as scission or crosslinking, as well as autooxidation. The mechanism(s) of action for hindered amine light stabilizers are not fully understood. The literature cites not only radical scavenging as a stabilizing mechanism, but also singlet oxygen quenching, and hydroperoxide decomposition.
A thorough description of ultraviolet absorbing and hindered amine light stabilizer chemistry as it pertains to coatings applications can be found in the Handbook of Coatings Additives; "Light and Heat Stabilizers for Coatings"; Dexter, M. and Schirmann, P. J., Marcel Dekker, Inc., 1987, pp. 225-269.
In addition to activity as a stabilizer, useful light stabilizers for coatings must have both compatibility with and/or solubility in the polymer and/or coating composition to be stabilized. Useful light stabilizers must also possess resistance to loss from the stabilized composition during normal processing and end-use applications. Many light stabilizers exhibit limited compatibility and/or solubility in certain polymers and coatings, and a tendency to exude, sublime and/or volatilize during weathering or exposure to elevated temperatures (all examples of fugitive characteristics), such as those employed in typical enamel cure systems.
One approach to overcoming the problems of fugitive stabilizers associated with compatibility, solubility, volatility, extractability, and migration of the light stabilizer has been to chemically bind the light stabilizer moiety to the polymer and/or the coating to be stabilized.
Methods disclosed in the art to prepare polymers with chemically bound light stabilizer group(s) include: (1) monomeric light stabilizers which can be polymerized or copolymerized (see U.S. Pat. Nos. 4,210,612 and 4,294,949); and (2) free radical initiators (peroxide and azonitrile compounds) containing light stabilizer functionality which can be used to impart the light stabilizer group to the polymer by free radical polymerization techniques (see U.S. Pat. Nos. 3,956,269 and 4,042,773; and U.S. patent application Ser. No. 87,473 filed Aug. 20, 1987, now U.S. Pat. No. 4,822,883 assigned to the assignee of the present invention and application).
Both methods have their drawbacks.
The monomeric light stabilizers are often difficult to prepare. In the case of the monomeric HALS, the polymerization or copolymerization has to be run under specific conditions in order to prevent oxidation of the hindered amine. Poor polymer conversion is often another limitation with the use of monomeric light stabilizers. This results in high levels of residual monomer which sacrifices product economics and quality. Moreover, in some cases the reactivity of the monomeric light stabilizers with other co-monomers is low, resulting in non-homogeneous distribution of the light stabilizer group along the polymer backbone.
In general, the effectiveness of the light stabilizer to protect the polymer and/or coating is dependent on the uniform distribution of the light stabilizer moiety throughout the polymer and/or coating.
Numerous types of light stabilizing compounds incorporated into free radical initiators are known in the prior art, for example, organic peroxide and azonitrile compounds containing hydroxybenzophenone, benzotriazole, and HALS functionalities. One problem with incorporating light stabilizers into polymerization initiators, such as peroxy and azo compounds, is that this approach limits the effective level of stabilizer that can be bound to the polymer at useful molecular weight production. The ability to adjust the concentration of the stabilizer bound to the polymer without adversely affecting the polymer molecular weight, as well as the molecular weight distribution is severely limited with the use of light stabilizer initiators. Moreover, azonitrile initiators in general possess low kinetic efficiencies, as low as 25%, thus limiting the percentage of light stabilizer that is chemically attached to the polymer. Further, in free radical polymerization, the light stabilizer group derived from the initiator is bound pendantly to the end(s) of the polymer chains. As a consequence, the distribution of the light stabilizer moiety on the polymer is non-homogeneous.
Another method of preparing polymer bound stabilizers is to attach the stabilizer group(s) to polymers which contain reactive functionalities. This method allows more efficient control over the stabilizer concentration, since the stabilizer is bound directly to the polymer or copolymer. In addition, homogeneous distribution of the light stabilizer throughout the polymer can be effectively controlled by the uniformity of the reactive functionalities along the polymer backbone.
U.S. Pat. No. 4,495,325 discloses ultraviolet light absorbing stabilizers containing hydroxyl functionality (such as resorcinol monobenzoate and dihydroxybenzophenone) which are chemically bound into an acrylic copolymer through glycidyl methacrylate.
U.S. Pat. No. 4,857,595 and U.S. patent application Ser. No. 84,608, filed Aug. 12, 1987 now U.S. Pat. No. 4,868,246 and both assigned to the assignee of the present invention and application, disclose hydrazide functionalized light stabilizers, both UVA and HALS hydrazides, which are chemically bound into polymers or copolymers through reactive anhydride groups contained along the polymer backbone. However, these applications do not disclose or teach the reaction of hydrazide with polymers containing hydroxyalkyl ester, carboxyl, epoxy, or isocyanate functionalities. Further, these prior applications do not reveal the use of two or more different functional groups on the polymer or copolymer. Moreover, the use of anhydride containing polymers would not be suitable for thermoset (enamel) coating applications. Hydroxyl functionality on the acrylic polymer is necessary to effect cure with conventional crosslinking cure agents such as melamine or isocyanate compounds.
Although various means are known in the art to attach light stabilizers to polymers, the use of light stabilizer hydrazides for producing polymer bound light stabilizer coating resins has not been disclosed.
The present invention overcomes the problems and limitations of the prior art.